This application claims the benefit of German Application DE 19858287, filed Dec. 17, 1998, herein incorporated by reference.
The present invention relates to a method and apparatus for contactless yarn monitoring in a spinning or bobbin winding machine and, more particularly, to a method for contactless yarn monitoring in a spinning or bobbin winding machine, having a sensor device for detecting measured values of a yarn moving in a yarn travel direction in which the moving yarn is subject to traversing motions, perpendicular to the yarn travel direction, in the measurement region of the sensor device, and to an apparatus for performing the same.
For monitoring the moving yarn in a spinning or bobbin winding machine, capacitive measurement systems, optical measurement systems, and measuring instruments that detect both optical measured values and capacitive values are employed. Such sensor devices are known for instance from European Patent Disclosure EP 0 423 380 A1.
In spinning and bobbin winding machines, traversing motions of the moving yarn are performed in a predetermined way, in order, for example, to create cross-wound bobbins, also known as cheeses, or to prevent the development of furrows in the surface of the typical rubber coating of the draw-off rollers that is meant to protect the yarn.
To suppress such motions of the yarn in the measurement region of a sensor device, the moving yarn, before and after the measurement location, is guided in a stationary position relative to the sensor elements by means of such stationary elements as yarn eyelets or yarn guide baffles, as shown for instance in European Patent Disclosure EP 0 423 380. A disadvantage of such yarn guide elements, however, is that by the contact of the yarn with the yarn guide elements, or the attendant friction, undesired influences or changes occur. For instance, the yarn surface becomes roughened by the eyelets or guide slits.
If the traversing motion of the moving yarn is not suppressed in the measurement region of sensor devices, then the constant change in position or spacing of the yarn from the sensor elements causes measurement errors, which can impair the reliability of the measurements and the adherence to the requisite yarn quality.
German Patent DE 26 02 465 C2 describes an apparatus in which a scattering disk for scattering diffuse light is used, the disk having a height that decreases toward the center, so that when the cross section or volume of a moving yarn is measured, more-reliable measuring results can be attained in a traversing region. If the yarn is traversing from the center toward the edges of the scattering disk, then in the process it increasingly covers the scattering disk with a greater length than in the center of the scattering disk, where the light intensity is higher. The utility of this apparatus is limited to systems that operate optically, with a non-homogeneous light intensity in the measurement region. Measurement errors that occur from changes of position of the yarn toward or away from the scattering disk are not compensated for with this known apparatus, and in a spinning or bobbin winding machine that has many winding stations, it is very complicated and expensive to equip each spinning station and to change the scattering disk manually. Also, the measurement errors that occur in capacitive measuring methods cannot be compensated for with this apparatus.
European Patent Disclosure EP 0 571 591 B1 describes an apparatus for yarn monitoring in which the yarn is monitored in the region of yarn traversing and moves back and forth between the sensor faces of the yarn monitoring apparatus in such a way that it moves toward and away again from the applicable sensor face. The traversing motion of the yarn is utilized for cleaning the sensor faces of the yarn monitoring apparatus, to counteract functional impairment of the measuring method by soiling. During the cyclical traversing motion, when the moving yarn has come quite close to the applicable sensor face, dust and fluff deposits are entrained from the sensor surface. Satisfactory measurement results are furnished by this yarn monitoring apparatus only as long as the yarn to be measured is moving in the measurement region of the sensor device within certain, quite narrow limits. The yarn monitoring device described in European Patent Disclosure EP 0 571 591 B1 is unable, or only inadequately able, to limit greater motions, such as those caused by the traversing motions of the yarn, that exceed these narrow limits. Therefore, measurement errors and considerable fluctuations in the measurement results, both of which impair the reliability and usability of measurement values for yarn monitoring, and can thus impair the yarn quality, must still be expected. The limitation of the yarn motion is accomplished by means of stops over which the yarn runs. The rounded edges and the use of wear-resistant material are indications of the considerable friction which is created by the movement of the yarn against the stops, which act as yarn guide elements, and which thus engender the above-described disadvantages of yarn guide elements.
It is accordingly an object of the invention to improve the monitoring of moving yarn upon changes of position of the moving yarn in the measurement region.
This object is attained according to the invention by a method for contactless yarn monitoring in a textile yarn winding operation. In such an operation, yarn is wound on a bobbin while being delivered lengthwise in a yarn travel direction toward the bobbin. At the same time, the yarn simultaneously executes traversing motions generally perpendicular to the yarn travel direction. According to the present invention, a sensor device is provided for detecting measured values of the yarn moving in the yarn travel direction. The sensor device has a measurement region within which the moving yarn is subject to changes of position. Position-dependent correction values are ascertained which correspond to measurement errors caused by changes of position of the yarn. The position of the yarn is monitored to determine at least one instantaneous position of the yarn, and the applicable measurement error is compensated for on the basis of the instantaneous position of the yarn.
The invention achieves a number of advantages. Compensating for the applicable instantaneous value of the measurement error as a function of the constantly monitored instantaneous position of the yarn in the measurement region assures the reliability of the measured values for yarn monitoring and thus assures the yarn quality regardless of the type of sensor device, for both optical and capacitive measurement methods. Values stored in memory can be used as standard values for other spinning stations and other spinning machines, as long as these spinning stations are structurally identical to the spinning station at which the measurements were made. The allocation and storage and memory of the values can be done in the form of a matrix.
Accurate, universally usable ascertainment of the yarn position in the measurement region is done by detecting the position of the yarn directly in the measurement region of the sensor device.
According to another aspect of the invention, the position of the yarn guide may be monitored in order to ascertain the change in position caused by the traversing motion of the yarn guide and to ascertain the applicable position of the yarn in the measurement region of the sensor device from the relationship between the position of the yarn and the position of the yarn guide, thus making a space-saving embodiment possible when only limited installation space is available at the sensor device. No additional expense for sensor parts is then necessary to detect the yarn position in this region. Both the location of the yarn guide in the cyclical course of the traversing motion and the location and geometric shape of storage brackets or guide baffles, which compensate for the difference in length between the oblique position and middle position of the yarn during the yarn guide stroke in order to prevent brief increases of yarn tension, should be taken into account in determining the positioning of the yarn in the measurement region. Each position of the yarn in the measurement region can be associated with a respective position of the yarn guide. Accurate determination of the position of the yarn guide in the measurement region is thus possible, without performing a detection in the measuring region itself. Determining the position of the yarn from the position of the yarn guide can be done by a mathematical method.
The empirical ascertainment of correction values from measurements with a comparison body at different positions, distributed within the measurement region of the sensor device, can be done quickly and simply. A reference yarn is expediently employed as the comparison body for the measurement.
According to a further feature of the invention, a sensor device is used for reference value ascertainment at a point along the yarn travel path where no traversing motion of the yarn is occurring. A measurement may be made to determine the correction value in each case at a first point along the yarn travel path where no traversing motion of the yarn is occurring and a second measurement may be made at a second point along the yarn travel path where the yarn is executing the traversing motion. The comparison of the measurement results from the two measurements, each made at the same location on the yarn, shows the position-dependent measurement error especially clearly. Thus, even measurement errors that are created by the sensor device itself, for example because of soiling or shifting, can be recognized and eliminated.
The invention can be used universally both with sensor devices that act simultaneously as yarn monitors and as yarn cleaners for correcting imperfections in the yarn, and with sensor devices in which a combination of optical and capacitive measuring methods is used for measured value detection. Such an embodiment of the sensor device is also space-saving, and by combining the functions, it lessens the expense for the required parts.
Another aspect of the invention provides that the sensor device further comprises a measurement gap which extends in the direction of the traversing motion of the yarn to prevent contact of the yarn with parts of the sensor device, and thus prevents undesired friction, even in the presence of a relatively major traversing motion of the yarn.
The invention makes it possible to obtain very accurate measured values even in the traversing regions of the yarn, without having to accept negative influences on the yarn surface from additional yarn guide elements for maintaining a certain measurement position of the yarn, and without undesirably limiting the traversing motion. Disposing a yarn monitor, which monitors the moving yarn contactlessly, in the yarn path either upstream or downstream from the pair of draw-off rollers, makes it possible, for instance, to wind bobbins with a very low winding tension.